The present invention relates to a process for recovering uranium and/or thorium from a liquid containing uranium and/or thorium which can recover uranium and/or thorium in high yield and can reduce a formed radioactive solid waste remarkably.
The process liquid waste discharged from uranium hexafluoride reconversion process contains 50-200 ppm of uranium and a very small amount of thorium. As a process for removing these radioactive materials from this process liquid waste, the applicant has disclosed a flocculation method using water glass as a flocculant in Japanese Patent Publication No. 38320 of 1973. This method is based on utilization of the characteristics of water glass (sodium silicate) that it becomes an amorphous silica, a polar adsorbent having a large surface area and a high activity, in an ammoniacal solution or an acidic solution and forms a precipitate (hereinafter is referred to as an amorphous silica precipitate) having an excellent filterability and can recover the uranium and/or thorium from the above-mentioned process liquid waste by making these radioactive materials captured by the formed amorphous silica precipitate.
In this process, however, the amorphous silica precipitate capturing, for example, uranium contains consequently 1-3% of uranium, but because of a low price of water glass, this amorphous silica precipitate has been stored up as a radioactive solid waste without any treatment and neither recovery of the captured radioactive materials from the amorphous silica precipitate nor regeneration of the amorphous silica to water glass have been carried out.
On the other hand, as a radioactive waste disposal method, there are at present proposed a sea dumping and a final storage. However, considering that these methods contain still many uncertain factors and are also inevitably very costly, a positive reduction of a formed radioactive solid waste as mentioned above is absolutely necessary and its realization has been strongly demanded.
Under these circumstances, the inventors have confirmed by the result of X-ray diffraction that the precipitate formed by the above-mentioned method using water glass as a flocculant is composed of an amorphous silica and have found out that this amorphous silica can be relatively easily dissolved in an alkali metal hydroxide solution to be regenerated to water glass. Further, they have found out that in the dissolution of the above-mentioned amorphous silica precipitate in the alkali metal hydroxide solution, when the amorphous silica precipitate is previously washed with water thoroughly, the dissolution of the amorphous silica precipitate in the alkali metal hydroxide solution becomes easier and the regeneration of the amorphous silica to water glass proceeds more effectively. They have also found out by their research on the ground of this phenomenon that anions such as NO.sub.3.sup.- contained in the amorphous silica preciptate act as an interfering factor in the regeneration of the amorphous silica to water glass, that is, that as in general an amorphous silica is a polar adsorbent, it does not adsorb anions in itself, but in this case the anions, dissolving in a water layer surrounding the amorphous silica, interfere with the regeneration of the amorphous silica to water glass.
Additionally, they have found out that in dissolving the washed amorphous silica precipitate in the alkali metal hydroxide solution, the amorphous silica can be more easily dissolved in a wet state than in a dried state.
The present invention has been devised on base of these finding of the inventors to meet the above-mentioned demand.